Recent developments in the field of radiation therapy have enabled increasingly precise control of the applied radiation dose. Dose profiles can fall off very rapidly over distances on the millimeter scale, enabling therapy to be applied close to critical anatomy, while subjecting such anatomy to acceptably low radiation exposure. This increased control of the applied dose has motivated increased attention to the digital imaging systems used to plan the therapy.
Magnetic resonance imaging (MRI) is of increasing interest for radiation therapy planning because it provides superior soft-tissue contrast to CT, enabling better visualization of pathological tissue relative to the healthy tissue. MRI, however, suffers from lower quantitative accuracy relative to CT, particularly in the area of geometric distortions. MR images have distortions that commonly reach several millimeters in magnitude.
It is possible to measure such distortions using phantoms and image analysis. Such techniques have been applied for many years over volumes pertinent to neurological scanning, using a Magphan® quantitative imaging phantom available from the Phantom Laboratory, Incorporated, located in Salem, N.Y. This phantom includes a large number of fiducial objects (markers or features) that can be located accurately within an image. Plastic spheres of 1-1.5 cm diameter are typically used as the fiducial objects. The relative location of each fiducial object is known ahead of time based on the design of the phantom, and that known location can be compared to the measured location of each fiducial object in the image. The difference characterizes the distortion throughout the volume covered by the fiducial objects.
A major challenge, however, is to perform such measurements over large, 3D fields of view that are pertinent to body imaging. A phantom large enough to cover such fields of view can weigh more than 100 pounds (45 kilograms), making it difficult to use in a clinical environment. The high weight is driven by the large volume and the need to fill the phantom with liquid in order to generate an MR signal. Classically, this is done with a very large phantom which can weigh over 100 pounds.
Others have addressed this challenge by applying a mathematical result that enables characterization of the geometric distortion throughout a volume based only on measurements of the distortion on a surface that surrounds the volume. This solution has several practical shortcomings:                1. The phantom is still heavy, presenting a challenge for safe handling by a wide range of clinical personnel.        2. The high weight of the phantom makes it difficult to add additional measurements beyond distortion, as this would further increase the weight. The currently available product performs only distortion measurements.        3. The measurements are performed with a pre-determined set of MRI pulse sequences, and do not always characterize the distortion that pertains to a specific pulse sequence used during clinical imaging.        
An alternative solution, not previously applied to this problem, is to construct the phantom of multiple, self-contained sections that are moved separately and reassembled on the imaging system patient table. Once reassembled, the sections are meant to function collectively as a single large phantom assembly or aggregate phantom.
A concern with this approach, however, is achieving the tight tolerances on the geometry of the assembled phantom sections. An accuracy of 0.5 millimeters or better is desired for the distortion measurements. It is extremely difficult to control the precision of the location of each phantom section relative to the other sections with such accuracy while enabling it to be re-assembled easily by the user. Further complicating the task, is the severe restriction on acceptable materials that can be used inside an MRI scanner to avoid safety issues and imaging artifacts.
Accordingly, there is a need for a practical method and apparatus to measure, with precision, geometric distortion of an imaging system, over large, 3D fields of view.
Maintaining acceptable levels of distortion in an MR scanner relies on properly controlling many conditions. It is critical to have a robust system of quality control for key imaging performance characteristics in order to detect significant deviations before they affect clinical operations.
Although an aggregate phantom may cover a large field-of-view, it may still not cover the entire field-of-view that may be of interest for some applications. Accordingly, there is a need for a technique to cover an entire field-of-view with a phantom that encompasses less than the entire field of view.
One common measurement performed on imaging systems is the spatial uniformity of the signal acquired. Uniformity can be a useful indicator for common failure mechanisms in subsystems such as an RF coil. The uniformity is usually measured by creating a large, uniform region of a phantom and studying the variation of the signal in an image of the uniform region. There are two common configurations used for this measurement:                1. A uniform region that covers only a single or a very small number (2 or 3) slices, and is oriented only in one direction, i.e., the measurement will support only slices oriented in one direction. In some imaging systems such as MRI, slices can be acquired in any orientation and so such a limitation is undesirable.        2. A phantom, often large, consisting of a uniform fill with no other features inside.        
There are also phantoms made of multiple sections, each with nothing inside except background fill. An assumption made in using such phantoms is that the fill in each section is the same as in all the other sections. This assumption can be unsettling, particularly for MRI, where the fill is fluid and the chemical properties can change over time.
Accordingly, there is also a need for an apparatus and method for measuring spatial uniformity of a signal acquired by an imaging system while compensating for signal differences attributable to different compositions or properties of different sections of a phantom.